Dhcpv4: Decouple from Ipv4

The code to set the ip address, netmask, and gateways from the
received offer moved to the direct Stackv4.
Stackv4: keep listening for DHCP offers
Merge branch 'dhcp-offers' of git://github.com/nojb/mirage-tcpip
Sync CHANGES
In the DHCP configuration thread, do not block indefinitely.

Terminate after the first offer is received, which brings us back
to the same functionality as before 4c7b0c7b1114fb71152fd8a95c
was applied.  Without this patch, TCP services never get a chance
to start because the configuration never ends.
Build Xen stubs separately, with CFLAGS from mirage-xen